In vitro approaches in evaluation and prediction of drug-drug interactions involving the inhibition of cytochrome P450 enzymes
Year: 2002
Type: dissertation
Abstract: In vitro systems have been widely used in evaluating potential drug-drug interactions in vivo.In the present studies, the effects of valproic acid, gemfibrozil, isoniazid, trimethoprim and sulfamethoxazole on the CYP forms ( CYP1A2, 2A6, 2C8, 2C9, 2C19, 2D6, 2E1 and 3A4) activities were examined in vitro using pooled human liver microsomes or recombinant CYP forms.The IC 50 and K i values were characterized using an apparent reversible inhibition.The K I and K inact values were characterized using a mechanism-based inhibition.The in vivo degrees of inhibition (i) were predicted in vitro using a scaling model: i = I/(I + K i ).In addition, the effects of human serum albumin (Hsa) and human liver cytosol (Hlc) on the in vitro enzyme kinetic estimates of the formation of hydroxytolbutamide, and on the inhibitory effect of gemfibrozil on tolbutamide hydroxylase activity were evaluated using human liver microsomes.Valproic acid preferentially inhibited CYP2C9 activity (K i = 600 µM), while it exhibited minimal or no inhibitory effects on the other CYP forms evaluated.Gemfibrozil inhibited CYP2C9 (K i = 5.8 µM), and CYP2C19 (K i = 24 µM) and CYP1A2 (K i = 82 µM) activities.Isoniazid was a mechanism-based inhibitor of CYP1A2, 2A6, 2C19 and 3A4 forms, with K inact values of 0.11, 0.13, 0.09, and 0.08 min -1 , and K I values of 285, 173, 112, and 228 µM, respectively.Trimethoprim (5-100 µM) selectively inhibited CYP2C8 activity (K i = 32 µM), while sulfamethoxazole (50-500 µM) selectively inhibited CYP2C9 activity (K i = 271 µM).Based on the aforementioned scaling model and the unbound plasma concentrations of the drugs, inhibition of CYP2C9 (7%) by valproic acid, CYP2C9 (56%), CYP2C19 (24%) and CYP1A2 (8%) by gemfibrozil, CYP2C8 (26%) by trimethoprim, and CYP2C9 (24%) by sulfamethoxazole would be expected in vivo.The addition of Hsa and Hlc to the incubation media distinctly changed the kinetic estimates of tolbutamide hydroxylation, with the predicted in vivo hepatic clearance (Cl h ) of tolbutamide hydroxylation (0.14 ml/min/kg) comparable to the actual in vivo value (0.15 ml/min/kg).However, the unbound K i of gemfibrozil for CYP2C9 (6 µM in the ABSTRACT 12 absence of Hsa and Hlc in the incubation media) was not markedly altered by Hsa (4 µM), Hlc (8 µM) or both Hsa and Hlc (9 µM) when the unbound substrate and inhibitor concentrations were considered.The results of the present studies indicated that in vitro, valproic acid reversibly inhibits CYP2C9 and gemfibrozil inhibits CYP2C9, 2C19 and 1A2; isoniazid is a mechanism-based inhibitor of CYP1A2, 2A6, 2C19 and 3A4; trimethoprim is a selective inhibitor of CYP2C8 and sulfamethoxazole of CYP2C9.In humans, the inhibition of these CYP activities by these drugs may result in significant drug-drug interactions, and can explain some of their documented drug-drug interactions.However, in most cases, confirmation of the predicted drug-drug interactions requires further in vivo experiments.The addition of Hsa and Hlc to microsomal incubations may yield enzyme kinetic estimates more comparable with in vivo results for CYP2C9 substrates.However, further experiments are needed to clarify the effects of Hsa and Hlc on other CYP substrates and other CYP enzymes.
Source: Työväentutkimus Vuosikirja
Author: Wen Xia
Language: English
Subfield: Pharmacology
Domain: Life Sciences
Open Access status: green
Funders: Helsingin Yliopisto, Oulun Yliopisto
OpenAlex ID: W2245459264